In ink jet printers, copying machines, facsimile, etc., an ink reservoir made of a polyurethane foam packed in an ink cartridge made of a thermoplastic resin is used in order to smoothly supply an ink (as disclosed in JP-A-5-38816 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")). The above cited publication discloses a three-dimensional network foam freed of foam cell membrane as a particularly preferred form of polyurethane foam. As a method for removing a foam membrane from a polyurethane foam there are disclosed a method which comprises dipping the polyurethane foam in an alkaline aqueous solution and a so-called explosion method which comprises setting the polyurethane foam in a pressure container, charging explosive gas like a mixture of hydrogen and oxygen and igniting in the mixture to remain only connecting strands of a coarse open-cell foam.
The three-dimensional network thus obtained by the three-dimensional networking treatment which comprises the removal of a cell membrane has a large volume void capable of reserving an ink and a small permanent compression set and thus exhibits a higher ink retention and a higher capability of maintaining its shape in the ink cartridge than other porous materials. Accordingly, even after a prolonged use, the three-dimensional network produces little space in the cartridge due to foam shrinkage, deformation, etc. and thus can supply a required amount of an ink as necessary.
However, a polyurethane foam which can decompose when acted on by water or a water-soluble solvent contained in an ink, such as ester-based foam is disadvantageous in that the decomposition product deteriorates the ink or lowers the surface tension of the ink, causing the printed ink to run. Further, foam pieces formed by decomposition can clog the ink feed opening.
On the other hand, an ether-based foam undergoes no decomposition due to a solvent contained in an ink but is disadvantageous in that unreacted components in the foam can elute with an ink to lower the surface tension of the ink, causing the printed ink to run.
Further, in order to pack an ink reservoir in the ink cartridge, the foam constituting the ink reservoir needs to have a proper rigidity and flexibility. After packed in the ink cartridge, the foam needs to maintain its original shape and hence produces no space in the ink cartridge. However, soft foams which have been frequently used have a deteriorated workability during packing because of excessive flexibility. Further, these foams leave much to be desired in the maintenance of shape.